Ramjet propulsion devices are known generally. Characteristics of ramjets have been utilized in numerous prior art devices. In general, a ramjet is a type of heat engine which utilizes the velocity of the device through the atmosphere to compress air within a combustion chamber. The air then mixes with a fuel, either solid or liquid, within a combustion chamber. In that the incoming air is compressed, its temperature is raised sufficiently high to initially combust the fuel which continues to burn due to the movement of air through the device. A suitably sized and shaped nozzle is provided at the rear of the combustion chamber and the heated air and products of the combustion pass through the nozzle forming a stream of high velocity gases. The thrust imparted to the projectile is thus the result of the high velocity gases which are exhausted from the nozzle at the rear of the projectile.
A well-known basic problem associated with the use of ramjet engines is that ramjets must operate above a speed of approximately Mach 1.5 in order to be efficient. Mach is a measure of velocity in that it is the velocity of the said object divided by the speed of sound in the ambient air conditions. In fact, ramjets experience their greatest thrust and efficiency in the range of Mach 1.5 to Mach 4.0. The most serious drawback of a ramjet is its inability to produce thrust at zero speed; hence, another method or device must be used in conjunction with the ramjet to provide sufficient initial velocity to the point where the ramjet can take over.
The simplicity and high thrust to weight ratio of a ramjet have cuased ramjet propulsion to be of significant interest as an assist to gun-fired projectiles. There is a strong current interest in a gun-fired projectile which has longer range than possible with explosive charges along. Ramjet propelled gun-launched projectiles are known and are capable of propelling a warhead far beyond the range of a conventional warhead.
Of interest in ramjet propelled projectile is U.S. Pat. No. 2,989,922 issued June 27, 1961 to M. Greenwood et al. for "Ramjet Propulsion Device". A number of slightly different embodiments of a ramjet projectile are disclosed. The device includes a supersonic diffuser at the forward end of the projectile for admitting air to a duct assembly. As the air enters the duct, adiabatic compression occurs causing an increase in temperature and a decrease in the velocity of air. The annular duct leads to the combustion chamber when combustion of the fuel occurs adding heat and mass to the already compressed air. Finally, a nozzle at the rear of the projectile converts some of the thermal energy of the hot gases into kinetic energy producing thrust. There is no provision in the ramjet disclosed in the device of Greenwood et al. to change the amount of thrust once the projectile is in flight.
Of general interest with respect to projectiles are U.S. Pat. No. 2,624,281 issued Jan. 6, 1953 to J. McNally for "Projectile" and U.S. Pat. No. 2,504,648 issued Apr. 18, 1950 to E. Chandler for "Projectile". Of general interest with respect to guidance systems that sense acceleration in U.S. Pat. No. 3,188,019 issued Jan. 8, 1965 to C. Boutin for "Simplified Internal Guidance System".